1. Field of the Invention
The present invention relates to a variable-resistance element (or device), the resistance of which is changed by a sliding contact moving on a surface of a resistor. In particular, it relates to a variable-resistance element having superior microlinearity.
2. Description of the Related Art
Variable-resistance elements having a sliding contact moving on a surface of a resistor require high wear resistance and longer operating life. Examples of conventional variable-resistance elements are disclosed in Japanese Unexamined Patent Application Publication No. 3-233904 and Japanese Patent No. 2889792 (corresponding to U.S. Pat. No. 5,475,359).
The variable-resistance element disclosed in Japanese Unexamined Patent Application Publication No. 3-233904 includes a resistor prepared by applying, by means of screen-printing, a resist paste composed of carbon black and carbon fibers dispersed in resin. Hard carbon fibers protruding from the surface of the resistor support the sliding contact, thereby preventing the resistor surface from becoming worn. The variable-resistance element disclosed in Japanese Patent No. 2889792 (corresponding to U.S. Pat. No. 5,475,359) has a resistor constituted from a lower resistor layer containing carbon fibers and an upper resistor layer composed of dispersed carbon black free of carbon fibers. A sliding contact of this variable-resistance element moves on the surface of the upper layer and is thereby prevented from becoming worn.
However, carbon fibers that constitute a resistor of a conventional variable-resistance element such as those described above are large, that is to say, they have a particle diameter in the range of 5 to 40 μm and a fiber length of 5 to 100 μm. Thus, the roughness of the surface of the resistor in terms of arithmetic average becomes larger, resulting in poor microlinearity. Microlinearity is an index that indicates the accuracy of variable-resistance elements.
FIG. 12 is a graph explaining microlinearity. The microlinearity is determined as follows. First, a sliding variable-resistance element having a sliding contact that can slide linearly with respect to the surface of the resistor is explained.
In the graph of FIG. 12, the ordinate indicates the output V of a sliding contact sliding on a resistor pattern when a rated voltage Vin is applied in the direction of the length L of the sliding contact, and the abscissa indicates the position X of the sliding contact on the resistor pattern. Based on the assumption that the resistance of the resistor is constant irrespective of the position, the change in output when the sliding contact is moved from a certain point on the resistor by a distance ΔX must be indicated by an ideal line P having an inclination Vin/L.
According to the ideal line P, the change in normal output when the sliding contact is moved from the point A to the point B by the distance ΔX is represented by ΔV=(ΔX/L)×Vin. However, the actual output S deviates from the ideal line P. As shown in equation (1) below, the microlinearity is determined by calculating the difference, i.e., VB−VA, between actual outputs VA and VB at the point A and the point B, respectively, determining the difference between VB−VA and the normal output displacement, and obtaining the ratio of the calculated difference to the applied voltage as a percentage. The closer the microlinearity is to zero, the higher the accuracy. A position sensor that requires high performance shows superior microlinearity in which the actual output S is close to the ideal line P.                     Microlinearity        =                                                            (                                                      V                    B                                    -                                      V                    A                                                  )                            -                                                (                                                            Δ                      ⁢                                                                                           ⁢                      X                                        L                                    )                                ⁢                Vin                                      Vin                    ×          100                                    (        1        )            whereinVA is the output value when the sliding contact is at the point A on the resistor;VB is the output value when the sliding contact is at the point B on the resistor;Vin is the voltage applied in direction of the length L of the resistor;ΔX is the distance between the points A and B; andL is the length of the resistor.Secondly, the microlinearity of a rotary variable-resistance element having a sliding contact rotatably attached to an arc-shaped resistor shown in FIG. 13 is determined by equation (2) below based on the same concept as that of the sliding variable-resistance element.                     Microlinearity        =                              (                                                            Δ                  ⁢                                                                           ⁢                  V                                Vin                            -                                                Δ                  ⁢                                                                           ⁢                  θ                                Θ                                      )                    ×          100                                    (        2        )            wherein Vin is the applied voltage;ΔV is the difference in output voltage between the measurement points (=VB−VA);Θ is the angle between electrodes; andΔθ is the difference between angles ∠A and ∠B (angle between the measurement points).